In some cases, you may need to prevent outbound SMTP mail while still allowing inbound SMTP mail. For example, a technical support service might want to receive e-mail inquiries that are answered only by telephone.
You can use several methods to prevent outbound SMTP mail from leaving the Exchange server, but this may be the easiest way:
When you remove the address information from the Internet Mail Connector, outbound SMTP mail doesn't have anywhere to go. Therefore, the Exchange server returns an immediate nondelivery report to the user, stating that the address is unrecognized.